There is known already, from U.S. Pat. No. 5,374,564 to Bruel, a method of fabricating thin films of semiconductor materials comprising the following steps:
1. bombarding a face of a substrate with ions, in order to implant those ions in sufficient concentration to create a layer of gaseous microbubbles forming microcavities defining a weakening layer;
2. bringing this face of the substrate into intimate contact with a stiffener; and
3. separation at the level of the layer of microcavities, by the application of heat treatment.
In the above document, the ions implanted in step 1 are advantageously hydrogen ions, but it is indicated that rare gases can also be used. As for the substrate, in the examples considered, it is formed of silicon, but it is indicated that it can also consist of semiconductors from group IV of the periodic table of the elements, such as germanium, silicon carbide or silicon-germanium alloys.
In the above document, separation is achieved by means of heat treatment, but it is then proposed, in variants of the method, to bring about the separation by applying, in association or not with such heat treatment, a detachment stress (for example, inserting a blade between the two substrates and/or applying traction forces and/or bending forces and/or shear forces, and/or applying ultrasound or microwaves of carefully chosen power and frequency). See in particular—U.S. Pat. No. 6,020,252 to Aspar et al. and improvements thereto.
The above technique has been tested on substrates formed of other materials, with implantation of other ions, light or otherwise.
The defects created in this way have been given various names: not only gaseous microcavities, but also flatter defects (sometimes referred to as microplatelets).
Generally speaking, this technology uses gaseous microcavities localized in the substrate, at a depth at which it is required to “cut off” a thin layer.
However, the principle of bombarding a substrate with ions is also known for other purposes. Thus it is known to use such bombardment, in the same field of semiconductors and microelectronics, to carry out doping, with efficacies in direct proportion to the implantation doses.
In this regard, various studies have been carried out with a view to characterizing accurately the consequences of such doping, i.e. to characterizing the crystal defects or inclusions that can result from such doping, and, where possible, to determining how to avoid or at least minimize such deterioration.
In particular, S. K. JONES et al., in the paper “Enhanced elimination of implantation damage upon exceeding the solid solubility”, Journal of Applied Physics, Vol. 62, No. 10, 15 Nov. 1987, pp. 4114-4117, discuss the implantation of ions of gallium or phosphorus, or even arsenic, in various substrates, including silicon, and come to the conclusion that, if the peak concentration of the doping impurity exceeds its solubility in silicon at the annealing temperature, there is improved elimination of defects of type II (consisting in particular of dislocation loops) when the precipitates, formed because of the excess implanted species, are dissolved. To be more precise, they found in particular that implanting gallium ions in silicon at 100 keV and at a dose of 1015/cm2 leads to amorphization of the silicon, with a gallium concentration peak that is greater than the solubility of gallium in silicon both at 900° C. and at 1100° C. (the limit of solubility of gallium in silicon is hardly of the order of 2.1019/cm3 at 900° C. or 5.1019/cm3 at 1100° C.); however, 16 hours of annealing at 550° C. leads to recrystallization of the amorphous phase, and if this is followed by annealing at 900° C. for 1 hour, precipitates are formed which dissolve if the temperature or the duration is increased (for example to 8 hours at 900° C.), eliminating defects of type II.
J. MATSUO et al., in the paper “Abnormal solid solution and activation behavior in Ga-implanted Si (100)”, Applied Physics Letters, Vol. 51, No. 24, 14 December 1987, pp. 2037-2039, discuss the effects of annealing on silicon doped with gallium, and come to the conclusion that the behavior on annealing substrates implanted with gallium is different from that observed with other dopants. To be more precise, they found that implanting gallium in silicon at 70 keV and 1015/cm2, followed by annealing for 10 s at 600° C., led to amorphization of the silicon, that the gallium concentration peak (2.1020/cm3) at 70 keV is ten times greater than the solubility limit of gallium in silicon at 900° C., and that annealing at 1100° C. for 10 s cause the gallium to precipitate.
Studies carried out to investigate the phenomena of formation of precipitates of gallium include that of S. DHARA et al., “Mechanism of nanoblister formation in Ga+ self-ion implanted GaN nanowires”, Applied Physics Letters, Vol. 86, No. 20, pp. 203199, 1 to 3 Sep. 2005; the authors found that implanting gallium in GaN nanowires at 50 keV and 2.1016/cm2 led to the appearance of precipitates of gallium with a diameter from 50 nm to 100 nm, and that there was a deficit of N atoms and an accumulation of Ga around the region of voids.
Note that gallium can be used in the field of semiconductors and microelectronics for applications other than doping, in particular synthesizing certain materials.
Thus M. K. SUNKARA et al., in “Bulk synthesis of silicon nanowires using a low-temperature vapor-liquid-solid solution”, Applied Physics Letters, Vol. 79, No. 10, September 2001, pp. 1546-1548, describe a low-temperature vapor-liquid-solid synthesis process that uses low-solubility solid metal solvents for silicon and other semiconductor materials. They propose in particular synthesizing silicon nanowires using gallium as solvent, building on a Ga—Si phase diagram that shows that there is a eutectic compound, Ga(1-x)Six with x=5.10−8%, which has a melting point of 29.8° C. (equal to that of pure gallium).